Accounts of explorations in the Great American Canyon, Placer County's Yosemite.

Thursday, January 17, 2008

A Visit to Green Valley

Yesterday I joined Ron Gould and Catherine O'Riley for a visit to Green Valley. The snow at the head of the trail had diminished to small patches, the sun was bright, and a stiff north wind aloft brought chill, turbulent breezes on and near the north rim of the canyon, where small vortices easily form, for the sunwarmed slopes below give rise to updrafts, and the updrafts collide with the north wind aloft, which skims directly across the summit of Moody Ridge ... and winds result which blow any which way. There was a singing of the wind in the groves of tall pines.

On the Green Valley Trail, we found very little in the way of storm damage from the recent cycle of storms. A branch here, a boulder there. The trail was in decent shape, sometimes tunneling through stands of manzanita, sometimes crossing open ridge crests with broad views. One watches, for instance, as Snow Mountain drops out of view behind Sawtooth Ridge. In fact, one can also see away up the Foresthill Divide to Tadpole Canyon and the (snow-covered) brushfields at the head of the Iowa Hill Canal, and a hair beyond, to the somewhat unremarkable spur ridge dividing the North Fork from New York Canyon. Yes, some fairly high country is in view at first. Then one drops too low, and the world shrinks to Green Valley itself, guarded on the west by the cliffs of Giant Gap.

We were on a mission to visit several old mining ditches. There are many old ditches in Green Valley, which is about a mile long, east to west, and a half-mile broad. The place lies directly in the Melones Fault Zone, which is marked by a narrow belt of peridotite and serpentine extending around a hundred miles from north to south. The bedrock in Green Valley is mostly serpentine. Faults place the serpentine in direct contact with completely different rocks to the west, in Giant Gap (late-Paleozoic "Calaveras Complex" metavolcanics), and to the east (a very narrow Mesozoic belt of mixed metasediments and metavolcanics, including marble, followed immediately by the miles-broad exposures of the "Shoo Fly Complex," early-Paleozoic metasediments).

So merely upon the basis of the bedrock we find some interesting geology in Green Valley. The faults bounding the Melones Fault Zone strike north and south and dip almost ninety degrees, that is, the fault zones are nearly vertical planes.

Of course they're not *really* planes. The fault zones curve back and forth, they shrink back and swell forward--but they are roughly planar.

And then, covering a lot of this interesting bedrock geology, are the Pleistocene sediments of Green Valley, rich in gold, rife in tunnels, with many many a mining ditch, to serve the many claims. They are glacial outwash sediments from several to many different periods of glaciation; but they are varied, and include a fascinating cemented conglomerate which cannot be a hundred thousand years old, one feels, but on the other hand, it is so incredibly cohesive--this mass of rounded pebbles and boulders and sand which, somehow, became so glued together it can stand firmly against the crushing floods of the river, so firmly it is worn into broad smooth surfaces like bedrock, not ripped apart pebble-by-pebble.

What is the cementing agent? This is not known. The "cemented outwash" is always, always, always found in direct contact with serpentine bedrock. So it is natural to deduce that some mineral leaching from the serpentine is the cementing agent.

Then there are much larger volumes of entirely uncemented, unconsolidated gravels, out-and-out river gravels, all Pleistocene in age, and including "exotic" boulders of many types of rock which not only washed directly down the canyon for many a mile (and what must have been the flood stages of the river which moved *those* big boulders!), but also including boulders of granite which very possibly originated in the upper basin of the South Yuba River, and were delivered into the North Fork by a glacier; since the North Fork robbed vast volumes of ice from the South Yuba basin during every major glaciation. The Yuba ice flowed right across the dividing ridge, from north to south, from the crest down to Blue Canyon.

Hmm. I should say that the rounded granite boulder is the quintessential glacial "erratic" left by stolen South Yuba ice. Although there is some granite in the upper North Fork, the basin is dominated by metamorphic rocks; whereas, the upper South Yuba is dominated by granite. So one can be somewhere in the North Fork, in Shoo Fly Complex bedrock, say, and walk through a forest growing on a body of glacial till of mostly South Yuba origins: and one sign of that provenance is that the boulders in the till are, mostly, granite.

There were many major glaciations. In fact, if a glaciation comes along, more major than many before, as may well be, it will tend to erase the moraines of all weaker predecessors. So many parts of the geologic record are missing. But who knows what might be deduced from the entire spectrum of Pleistocene gravels in Green Valley? A careful radiogenic dating, and a careful petrological analysis, of these sediments might reveal traces of a dozen different glaciations. The glacial sediments can be found as much as 600 feet, perhaps 700 feet, above the present river. This betokens age. Paleontology might even enter the picture, in the study of these interesting gravels.

Our first ditch took its water from Pyramid Ravine, at the west end of Green Valley, heading on Moody Ridge, and flowing from north to south. It led the water into the Vale of the Pyramid, or whatever you call that lovely little swale along the High West Trail, as you descend towards Cedar Meadow, at the foot of the Vale. Directly above Cedar Meadow to the west stands the summit of The Pyramid, a serpentine knoll of elevation 2277', thus 477' above the river, which flows another quarter-mile or so south. We took a brief look at the brush-infested ditch, of fairly large proportions, for such a small stream, and then dropped back into the Vale and traversed along south until we could climb to the crest of the Pyramid Spur, a ridge dropping from the summit of Moody Ridge, over two thousand feet, to the river. We followed the ridge crest south to The Pyramid, where a gnarled Canyon Live Oak has an old horseshoe draped around one branch, and now deeply embedded in the wood.

We dropped down the ridge to the south, thrashing through some brush before entering the more open coniferous forest, below, exactly where the Pleistocene gravels pick up, and the serpentine ends. Here we found another old ditch of modest proportions, and followed it east to Pyramid Camp, where a depressing amount of garbage still hasn't been carried up and out. A few steps further brought us back to the West Trail. We followed this north, away from the river, reaching Cedar Meadow, with its piped spring, which needs work, and turning east onto the Low West Trail. This led us onto the High Ditch Trail, quite a remarkably nice trail, which follows along near the 2080' contour from near Cedar Meadow on the west, all the way to the Iron Point Trail, on the east, the sinuous and even angular curve of its length measuring over a mile.

From the eastern terminus of the High Ditch we dropped back towards the North Fork, south, on the Iron Point Trail, which itself branches from the Euchre Bar Trail, until we reached the Green Valley Blue Gravel Mine (GVBGM) ditch. We followed this large mining ditch to the south and east until we broke out upon the cliffs at the east end of Green Valley, near cliffs of pure marble rising steeply from the river. Here we had our lunch, sitting in warm sun, while enjoying a fine view down the river.

Next we followed the GVBGM canal the other way, back to the Iron Point Trail, and then continuing to the terminus, where the ditch widens into a small reservoir. Here one can continue west, crossing Moonshine Ravine and finding yet another ditch, leading west, which brings one immediately to the High East Trail. There are actual several ways to get back and forth from east to west in Green Valley. The mining ditches make wonderful trails. Both the High Ditch and the GVBGM, slightly below the High Ditch--oh, a hundred yards at least are between them--both pass through the mysterious patch of Valley Springs rhyolite ash boulders, lying mainly on the east of Moonshine Ravine. There must be several acres of ground covered with this deposit. Some of the boulders are eight feet through, maybe more. They have been weathering and eroding for a long time, and show deeply pitted and hollowed surfaces, sometimes ribbed, along multiple layers of rhyolite ash, which differ in hardness or integrity, slightly, somehow. So. This is a sedimentary deposit, I am willing to hazard, which is not derived from the North Fork, and is not really glacial outwash, or, at least, not the usual sort of glacial outwash.

The "area of rhyolite boulders" masks the serpentine indefinitely far beneath, but certainly less than one hundred feet down, I would say. The rhyolite "sweetens" the soil, as it were, and there is an unusual meadowy aspect there.

So, we followed various mining ditches, and obscure trails, back and forth and up and down in Green Valley. As the shadows lengthened we started back up on the East Trail. We had gained the shelter of the canyon, coming down the trail: for the sharp breezes had been stilled, and the January sun was comfortably warm. But, as we climbed back up the trail we left the shelter of the canyon depths. At first, I noticed a singing from the pines atop Moody Ridge, a thousand feet above me. Th North Fork could also be heard roaring along quietly a thousand below, to the south. "Odd, to hear that steady river of wind in the trees, from such a distance," thought I. But the mystery was soon solved, for I climbed right up into the wind. It steadily strengthened as I steadily climbed.

However it was nice to be wind-cooled while climbing the old trail. And it was a very nice day in Green Valley.